We will determine the basis, at the molecular level, for the apparent limitation of R-plasmid bacterial host range among hospital bacteria. Preliminary experiments indicate the presence of a migrating controlling DNA sequence analagous to that proposed for higher life-forms. This element will be further characterized by genetic and molecular parameters. The verification of the modus operendi of the proposed controlling element has important implications to the understanding of the epidemiology of infectious antibiotic resistance among Gram-negative bacteria common to the hospital environment.